Electrostatic printing apparatus



Sept. 5, 1967 s. B. M FARLANE 3,339,469

ELECTROSTATIC PRINTING APPARATUS Original Filed Aug. 6, 1962 2$heets3heet J INVENTOR. SAMUEL B. M: FARLANE Sept. 5, 1967 5 B -MCFAR[ANE 3,339,469

ELECTROSTATIC PRINTING APPARATUS Original Filed Aug. 1962 2 Sheet $hee1,2

INVENTOR. SAMUEL B. MC FAELANE.

United States Patent 3,339,469 ELECTROSTATIC PRINTING APPARATUS SamuelB. McFarlane, Summit, N.J., assignor to Sun Chemical Corporation, NewYork, N .Y., a corporation of Delaware Original application Aug. 6,1962, Ser. No. 214,950, now Patent No. 3,220,833, dated Nov. 30, 1965.Divided and this application Aug. 23, 1965, Ser. No. 481,624 5 Claims.(Cl. 95-1-7) This invention relates, in general, to the field ofelectrophotography and has as an objective the improving of xerographicand electrostatic printing techniques to provide image transfer by aforce field rather than through mechanical pressure to yield uniformprinting on various materials and even corrugated surfaces and as acontinuous printing operation when desired. The present invention is adivision of US. Ser. No. 214,950, filed Aug. 6, 1962, entitled,Electrostatic Printing Method and Apparatus, by the same applicant.

In the usual process of xerography as disclosed, for example, in CarlsonPatent No. 2,297,691, issued Oct. 6, 1952, a photoconductive insulatingmaterial provided with a conductive backing is given a uniform electriccharge over its surface and is then exposed to light or radiationthrough an image to form an electrostatic latent image on the insulatinglayer. The electrostatic charges making up the image are formed due tothe light exposure discharging photoconductive surface areas to theconductive backing plate in accordance with the light intensityimpinging upon the incremental surface areas. Development of the imageis effected by bringing oppositely charged printing particles to theelectric charge of the pattern itself such that they are held thereonelectrostatically in the outline of the electrostatic latent image.Thereafter, the developed image is transferred toa suitable material towhich it may be fixed if required. In other embodiments, theelectrostatic latent image is transferred from one non-conductivesurface to another before it is used for printing purposes.

Owing to the internal electrostatic lines of force established in thelatent image due to the charge distribution, the prior art systems haveexperienced great difiiculties with the development step in attemptingto overcome concentrated charges which result in the copies reproducedhaving hollow centers and emphasized edges. Usually some form of adevelopment electrode is required. Such electrodes have at least asurface of a conductive material biased slightly or held at the samepotential as the plate or backing conductive member to establishelectrostatic fields outwardly of the image and away from its backingplate to minimize the distortion.

The present invention essentially avoids these problems by providing astrong development field with definite lines of force establishedthrough an air gap between at least the image and the transfer medium ormaterial to be printed. The problem is further minimized by avoidingintermediate transfers of the electrostatic latent image, therebyeliminating handling of the charged images which heretofore causedsmudging.

By using an electrostatic latent image, the present invention eliminatesstencils and hence permits continuous printing of repetitive patterns orimages of printed or coated conductive screen of very fine mesh as thephotoconductive electrostatic image carrier. The non-conductive coatingof the screen is uniformly charged and a light image focused thereon toproduce an electrostatic latent image, defined by charges along theboundaries of the interstices. The discharged areas lose their chargesthrough the light-sensitive, non-conductive coating to the internalscreen wire conductors. This pattern is suitably dusted with a pigmentin the form of a fine printing powder, preferably, from the face of thegrid which was not exposed to the light pattern, thereby requiring theparticles that will adhere to the electrostatic latent image to passthrough the screen mesh before they can cling to the charged boundariesof the interstices in accordance with the image pattern thereon. Thisenables close control of powder size to produce fine printing. Thedusted image is then placed in or caused to enter a strong electrostaticfield developed in an air gap, which space also receives the transfermedium. The printing particles are projected across the air gap andstick to the transfer medium, being subsequently fixed thereto, ifnecessary, to provide permanent copy. By properly arranging the fieldpolarity relative to the charge of the particles, they are actuallyrepelled from the grid and attracted by the other field bounding platebut are intercepted by the intermediately disposed transfer medium.

The foregoing apparatus may operate in a continuous manner or instep-by-step fashion for individual image reproduction. In the former apattern is printed on a fabric or a film of images is reproducedcontinuously. In the latter, images are reproduced intermittently, as inoffice business machines and the like.

Other embodiments include the use of thin metal foils coated withphotoconductive material as the electrostatic latent image carrier; theuse of image carriers in the form of circular drums or endlessconveyors; and the use of separate electrostatic field forming meansindependent of the image carrier.

One unusual feature presented by the air gap charge transfer arrangementpermitting printing on rough or corrugted paper is the facility tocreate an illusion of depth. Embossed or specially indented paper can beprinted in the indentations to present the appearance of a threedimensional scene.

With the foregoing in mind, it is an object of the invention to providea novel printing apparatus capable of continuous printing from projectedimages without pressure contact with the printing material.

A further object is the provision of such an apparatus which minimizesthe mechanical elements and relies upon electrostatic transfer throughan air gap to the transfer medium without the necessity of wettingagents.

It is a still further object to provide an arrangement whereinelectrostatic latent image techniques are employed in a contact-free gaputilizing a particle size limiting image carrier.

In the drawing:

.FIG. 1 is a schematic view partly taken in side elevation of apparatuscapable of employing an electrostatic image in an electrostatic field toproject powdered pigment to a. transfer medium to print the imagethereon;

FIG. 2 is a view of a portion of the structure of FIG. 1 in frontelevation;

FIG. 3 shows a portion of the electrostatic image-forming grid of thestructure of FIGS. 1 and 2 in plan and greatly enlarged;

FIG. 4 is a view partly in section of the structure of FIG. 3 takenalong the plane 4-4 and looking in the direction of the arrows assignedthereto;

FIG. 5 shows a modification of a portion of the structure of FIG. 1; and

FIG. 6 is a view in plan of the electrostatic imageforming grid of FIG.3 but showing an electrical connection for the modification of FIG. 5.

Referring now to the drawing, and particularly to FIGS. 1 and 2, anendless conveyor is generally shown at 11, as comprising a coated screenforming a grid 13 supported by driving sprockets 15, 16, 17 and 18 forendless rotation. A common drive is provided through belt 20 by way of apulley 21, further belt 22 and motor 23.

The grid 13 is made up of flat or round screening, shown as theindividual conductive wires 24 in FIGS. 3 and 4. The wires 24, beforebeing formed into screen, are coated with a light-sensitivephotoconductive coating 26. The screening wires 24 may be of anysuitable, flexible conductive material, such as, conductive nylon ornickel screen covered with a thin layer of photoconductive material,such as, zinc oxide in an organic resin binder, which is commerciallyavailable. The coating is capable of being charged relative to thescreen and holding the charge for at least a short period so long as itis retained in a dark or dimly lighted area, such as an enclosure (notshown) for the apparatus of FIG. 1.

A fur or plush roller 23' is continuously driven by a motor 28 in lightcontact with the grid 13 to distribute a uniform charge over itssurface. A small metal rod 25 lightly engages the roller 23 and isgrounded to drain electric charge from the plush. With the endlessconveyor 11 being driven in the counterclockwise direction shown, thecharged grid 13 is presented to an electrostatic imageforming areaestablished by the light projection arrangement 27.

When a continuous printing operation is desired, such as in the case ofprinting patterns on fabrics, printing Wall paper, or the like, anendless loop pattern 29 is used to project images onto the grid 13. Aslit exposure mechanism comprising projection means, including lens 30,an image slit 32 and projection slit 34 are provided so that the lensprojects through the image slit a focused image of the projection slitin conventional manner. The pattern 29 is passed across the projectionslit 34 at the desired rate of speed by the sprocket drive means 36,which is, of course, synchronized with the rate of travel of the grid13. Latent electrostatic images indicated by the small 26 are producedon grid 13. These images are composed of charges distributed along thesurfaces of the non-conductive coatings 26 such that the boundaries ofthe interstices actually define the image charge 31.

A dusting chamber 41 is shown disposed adjacent to the interior surfaceof the endless conveyor 11 with its shield 41 opposite thereto such thata cloud of dust is developed by the contained rotating brush and appliedto the grid 13 and those'particles which are of sufficiently finegranular size are attracted through the interstices of the grid 13 andstick to the oppositely charged areas of the electrostatic latent image,thereby developing the same. The dust particles may comprise powderedink of a thermosetting variety or other similar printing mediums asmentioned or commercially used.

The now dusted latent iage 31 enters an electrostatic field of arelatively high value compared to the latent image charges. This fieldis formed between the screen, composed of wires 24, all joined inelectrical connection by the strip 45 (FIG. 3), and a flat electrode 47(FIG. 1) spaced therefrom to form an air gap. The electrical potentialdifference is of the correct polarity to project the powder imageparticles through the air space toward the fiat conductor 47. However,these particles are intercepted in their flight by the transfer medium49 moving in synchronism with the grid 13 through the supply roll drive51. The medium 49 is maintained in contact with or adjacent to flatplate conductor 47.

The driving mechanism for the printing medium 49 and the endlessconveyor 11 are synchronized such that the printing surface moves at thesame speed through the electrostatic field as the electric image. Thespacing in the air gap (of necessity illustrated as grossly exaggerated)is made quite small in order that a relatively small voltage differencemay establish sufficient electrostatic force lines for fine detailprinting. An air gap of several hundredths of an inch between theconveyor 13 and the printing surface 49 requires a voltage difference ofseveral hundred volts (preferably direct current) between backing plate47 and the conveyor internal screen comprised of the wires 24, forproper printing. Factors affecting the optimum printing voltage are:width of gap, humidity, type, pigment, and paper. The gap Width may bevaried as much as 0.1 inch for voltage dififerences of 1500 volts.

It is believed that ionization of this air space is not necessary butthat the charged powdered ink particles are simply projected through theair space to impact the printing surface, the latter naturally beingtaut to maintain a substantially parallel relationship with the conveyor13. The parallel relationship is not essential to printing by thisapparatus but the spacing should be uniform to preclude arcing at anarrow point, and the illustrated apparatus provides uniformity.

The so-transferred image is then fixed, if necessary, as by the heatingcoils 55 (when thermo-setting ink is used) located along the printingsurface path of travel. For the screen, comprising the wires 24, a meshsize of approximately 200 per inch to 325 per inch is satisfactory forfine printing. Also, a flat screen of electro-formed nickel havingapproximately 200-250 strips per inch may be coated, as mentioned toform a suitable image carrier.

In the embodiment of FIGS. 5 and 6, the same arrangement of FIGS. 1 and2 is employed except that the screen formed by the wires 24' of theconveyor 13 is grounded as shown by the slider 61 which frictionallycontacts the common strip 45' and the electrostatic field is formedbetween the flat conductor plate 47 and a second conductive plate 63.Similarly, the powder duster chamber 41' is shown effective at the outersurface of the conveyor 13 such that particle filtration is notachieved, However, heavier printing results from the greateraccumulation of the powdered ink on the electrostatic latent image whichis formed in the manner heretofore explained on the endless conveyor13'. When the image enters the field, the powdered ink is projectedtoward the plate 47' and the paper (shown as corrugated) or othermarkingsurface 49' intercepts it for printing purposes. Also, at theexpense of losing the powder filtration feature, the endless conveyorgrid 13' may comprise a thin flexible aluminum foil or film coated withthe photoconductive layer replacing grid 13' to permit electrostaticimage formation thereon. In such an arrangement the screening or grid isdispensed with entirely along with plate 63 and the foil actuallybecomes the only other electrode for use with the plate 47' to form theelectrostatic printing field. Also, in the structure of FIG. 5, theconductive plate 63 can be aluminum foil or the like formed as a backingfor the entire conveyor 11'.

In all embodiments, the usual fine pigment printing powders such as drydye or carbon black are suitable. Such inks are suitably selected topermit printing on the various materials mentioned.

It may be appreciated that any of the embodiments disclosed is adaptableto continuous or intermittent operation, it being only necessary tocorrelate the mechanical driving mechanism after well-known principles.Also, the apparatus herein disclosed is capable of producing xerographicplates which can, of course, be used in or as a part of an electrostaticfield for printing in the manner al-- ready described.

It should be apparent that the endless pattern master film 29 can aseasily comprise a film strip which carries images to be reproduced Also,the apparatus can function as an oflice reproducing machine by simplyusing a holder for the document to be reproduced at the proper locationfor focusing an image of the entire document on conveyor 13. In thisarrangement, however, the device is preferably intermittently operable.The electrostatic latent image is produced ducing a dwell time, as isalso the printed copy. Thus the apparauts including the conveyor iscycled to charge the conveyor image receiving area; to arrest conveyormovement for a light flash of the image onto its charged surface; tostart conveyor movement to powder and deliver the image to the fieldproducing area; to establish the field during this dwell time to printand to move the thus printed copy through the fixing station and out ofthe machine. Suitable intermittent type drives are well known to achievethe foregoing described operation and are therefore not illustratedherein.

While the present invention, as to its objects and advantages, has beendescribed in relation to certain specific embodiments herein, it isunderstood that it be intended that the invention be construed withinthe spirit and scope of the appended claims wherein:

What is claimed is:

1. In apparatus for electrostatic deposition printing, the combinationof an image carrier in the form of a grid; means for forming anelectrostatic latent image on the grid; means for powdering the imagewith marking material through the grid; means -for establishing anelectrostatic field having an air gap; and means for introducing thepowdered image into the air gap along with a transfer medium inspaced-apart relation whereby the marking material is projected throughthe air gap to reproduce the image on the transfer medium.

2. In apparatus for electrostatic deposition printing, the combinationof an image carrier in the form of a conductive screen having aninsulating photoconductive coating; means for uniformly charging thecoating; means for forming an electrostatic latent image on the imagecarrier coating; means supporting the carrier for endless rotation;means for powdering the image with marking material; means forestablishing electrical connection to the conductive screen; aconductive plate spaced from the carrier; means for establishing anelectrostatic field between the plate and the screen having an air gapbounded on one side by the screen and on the other side by the plate;means for introducing the image carrier with the powdered image into theair gap along with a transfer medium in spaced-apart relation wherebythe marking material is projected through the air gap to reproduce theimage on the transfer medium; and means for fixing the marking materialon the transfer medium.

3. In apparatus for electrostatic printing, the combination of aconductive member having a light-sensitive coating thereon; means forproducing an electrostatic latent image on said coating; means forapplying granular printing material on the image; a planar conductivemember disposed in spaced-apart relation with the conductive member;means for introducing a transfer medium into said spacing; and, separateconnections to the conductive member and the planar conductive memberwhereat a potential difference may be applied to project the printingmaterial in the direction of the planar conductive member but onto thetransfer medium to print said image.

4. In apparatus for electrostatic printing, the combination of aconductive screen having a photoconductive light-sensitive coatingthereon; means for producing an electrostatic latent image along theinterstice boundaries of said coating; means for applying oppositelycharged granular printing material to the image; a planar conductivemember disposed in spaced-apart relation with the conductive screen;means for locating a transfer medium in the spacing area; and, separateconnections to the conductive screen and the planar conductive memberwhereat a potential difference may be applied to project the printingmaterial in the direction of the planar conductive member but onto thetransfer medium to print said image.

5. In apparatus for electrostatic printing, the combination of aconductive screen supported for rotary movement; a light-sensitivecoating on the screen; means for producing an electrostatic latent imageon said coating as it is moving; means for applying granular printingmaterial on the image through the screen interstices; a conductivemember in spaced-apart relation with at least a portion of the screen; atransfer medium disposed in spacedapart relation with the screen and theconductive member whereat a potential difference may be applied toproject the printing material in the direction of the planar conductivemember but onto the transfer medium to print said image.

References Cited UNITED STATES PATENTS JOHN M. HORAN, Primary Examiner.

1. IN APPARATUS FOR ELECTROSTATIC DEPOSITION PRINTING, THE COMBINATIONOF AN IMAGE CARRIER IN THE FORM OF A GRID; MEANS FOR FORMING ANELECTROSTATIC LATENT IMAGE ON THE GRID; MEANS FOR POWDERING THE IMAGEWITH MARKING MATERIAL THROUGH THE GRID; MEANS FOR ESTABLISHING ANELECTROSTATIC FIELD HAVING AN AIR GAP; AND MEANS FOR INTRODUCING THEPOWDERED IMAGE INTO THE AIR GAP ALONG WITH A TRANSFER MEDIUM INSPACED-APART RELATION WHEREBY THE MARKING MATERIAL IS PROJECTED THROUGHTHE AIR GAP TO REPRODUCE THE IMAGE ON THE TRANSFER MEDIUM.